KISSsys application: Lifetime analysis of a 4MW wind turbine gearbox

Lifetime analysis of a wind turbine gearbox using KISSsys
KISSsys application:
Lifetime analysis of a 4MW wind turbine gearbox:
Two planetary stages plus one helical stage
1 Task
In order to compare different gears with respect to their lifetime in short time, a model of a complete
gearbox including all relevant parameters shall be established. The lifetime analysis for the gears,
shafts and bearings shall be performed for a user defined load spectra (speed and torque versus time on
the input shaft). The results are summarized in a report.
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2 Solution
Using KISSsys the complete gearbox, consisting of two planetary stages and one helical stage is
modeled. Hence, the kinematics of the epicyclic gear is known and the loads on the gears, bearings and
shafts can be calculated automatically. Using these loads, the lifetime of the mechanical elements is
automatically calculated using KISSsoft in the background.
It is hence sufficient to define the speed and torque acting on the input shaft in order to analyze all
mechanical elements present.
The lifetime analysis of the elements is performed in KISSsys using a damage accumulation algorithm.
Calculating a damage equivalent load based on the load spectrum is not meaningful in this application
since the slope of the S-N curve of the bearings and gears is not equal. The load spectra can either be
defined manually or read from a text file.
Since all analysis parameters and results are stored in variables, comprehensive reports can be
generated.
3 Description of the model
3.1 Structure of the gearbox
The gearbox consists of two planetary stages and one helical stage.
Technical data of the gearbox (approximate):
Power rating
4MW
Nominal torque
2500kNm
Nominal speed
15Upm
Reduction 1:120
Diameter
2000mm
Length
2800mm
Lifetime required 20years
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Figure 3.1-1 Structure of the gearbox: Black: shafts, blue: gears, Yellow: bearings, red: couplings. Note that only
one planet per stage is shown.
The two rings of the planetary stages are fixed, the first stage has five, the second three planets (only
one planet is shown in the 3D view). The constraint of the rings is modeled in KISSsys by setting the
rotational speed to zero. The power flow of the gear box is shown below:
Figure 3.1-2 Schematic of the gearbox with power flow (or forces) (red), mechanical elements (shafts, gears,
couplings, bearings) (black) and power input and output.
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The three dimensional representation and schematic shown above are based on a tree structure shown
below:
Figure 3.1-3 Tree structure of the gearbox consisting of several shafts, power input
and output and connections. For the sun shaft of the second stage, the elements
arranged on the shaft (bearings and gears) are shown.
3.2 Definition of forces acting
The user sets the nominal speed and torque on the input shaft (rotor hub)
using the following dialog:
Figure 3.2-1 Dialog for definition of input (or output) speed and input (or output) torque
These nominal values can be modified using a load spectrum (frequency, torque and speed). The load
spectrum can be defined manually or read from a text file. The spectrum again is selected / defined
using the following dialog:
Figure 3.2-2 Selection of the applicable load spectrum through a
KISSsys dialog and windows open file dialog
The load spectrum is then read and stored in KISSsys as
a table:
Figure 3.2-3 Load spectrum read in KISSsys. The speed
remains constant on the nominal value. The left column shows
the frequencies (sum of all frequencies=1), middle column
shows the relative torque
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Furthermore, the user can define lubrication parameters such as lubrication mode, lubricant and
lubricant temperature:
Figure 3.2-4 Selection of lubricant, temperature and lubrication mode
3.3 Execution of analysis, summary of results
In the user interface (see below) after having defined the loads, spectrum and lubrication, the user can
start the lifetime calculation on calling the function „Calc. Lifetime“.
Figure 3.3-1 Overview of the KISSsys interfaces (user interface: large table in top right corner)
This function calculates for each step of the load spectrum a damage for all mechanical elements
present using the respective S-N curves (which are all different for all parts). The damage is
accumulated continually and being transformed into a lifetime at the end of the analysis. The analysis
of the gears is according to DIN, ISO or AGMA standards, the analysis of the bearings according to
standard L10 calculation or DIN/ISO 281.
The tooth geometries can be modified and optimized through calling the respective KISSsoft
interfaces.
Analysis of a gearbox with modified tooth data takes a few seconds only.
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Figure 3.3-2 Optimization of tooth form using KISSsoft, automatic exchange of data between KISSsoft and KISSsys
ensures that the analysis of the gearbox uses the correct tooth geometry.
4 Conclusion
The great advantage of this systematic approach to gearbox analysis is the gain in time when
comparing different gear designs or when performing parametric studies.
While the modeling of such a system does require time and knowledge of the software, the time saved
afterwards when optimizing the gears and for reporting is great, especially if the analysis is to be
performed for load collectives with a high number of steps.
The reporting functions delivering comprehensive reports for all members analyzed helps a lot when
documenting the analysis and the design of the gearbox respectively. The not so well liked work of
writing analysis reports is accelerated considerably.
The model shown above can be extended to other epicyclic gears. A possible next step would be to
introduce additional forces and bending moments on the input shaft, greatly affecting the bearings
lifetime.
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